Fisher, A., Davis, E.E., and Escutia, C. (Eds.), 2000 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 168
6. RELATIONS BETWEEN TEXTURAL CHARACTERISTICS AND PHYSICAL PROPERTIES OF SEDIMENTS IN NORTHWESTERN CASCADIA BASIN1 Amanda Cavin,2 Michael Underwood,2 Andrew Fisher,3 and Aaron Johnston-Karas3
INTRODUCTION Sedimentary deposits of Cascadia Basin lap onto the Juan de Fuca Ridge to within 20 km of the spreading axis (Davis and Currie, 1993). Collectively, these interbeds of hemipelagic mud, mud turbidites, silt turbidites, sand turbidites, and debris-flow deposits act as a relatively low-permeability barrier that inhibits the hydrothermal connection between underlying igneous crust and the overlying reservoir of ocean water. The primary purpose of Leg 168 of the Ocean Drilling Program (ODP) was to explore the causes and consequences of ridgeflank hydrothermal circulation (Shipboard Scientific Party, 1997c). One important aspect of this overall goal was to determine how changes in thickness of the sediment cover affect heat flow, fluid flow, fluid composition, and chemical alteration of the igneous crust. Fluid circulation through the sediment is influenced by a variety of textural parameters and intrinsic physical properties. Physical properties of sediments usually change in a predictable way with increasing depth and mechanical compaction, but different lithologies display different compaction gradients. During Leg 168, 10 drill sites were organized into three transects. The Hydrothermal Transition Transect is located closest to the ridge crest and includes Sites 1023, 1024, and 1025 (Fig. 1). The total thickness of sediment above igneous basement ranges from 192.8 m at Site 1023 to 97.5 m at Site 1025. The Rough Basement Transect is located ~100 km from the ridge crest and includes Sites 1026 and 1027. Sediment thickness there ranges from 228.9 m above a basement high (Site 1026) to 606.2 m above an adjacent basement low (Site 1027). The Buried Basement Transect begins 40 km from the ridge crest, above a basement high, and extends approximately 35 km to the east. Sites 1030 and 1031 are located above the basement high and contain less than 45 m of sediment (Fig. 1). Sedimentary successions at Sites 1028 and 1029 reach thicknesses of 132.5 m and 220.1 m, respectively. Site 1032 was used primarily as a logging site and was not sampled as part of this study. Shipboard scientists subdivided the sedimentary succession throughout the study area into three principal lithofacies units and subunits (Fig. 1). In general, these sequences coarsen and thicken upward from a basal interval of hemipelagic mud through a unit of mud and silt turbidites into a unit of mud, silt turbidites, sand turbidites, and debrisflow deposits. The sediment index properties (bulk density, water content, porosity, and void ratio) were measured aboard the JOIDES Resolution (Shipboard Scientific Party, 1997b, 1997d, 1997a). Shorebased work was devoted to accurate measurements of grain-size parameters. To allow for valid cross-correlation, the samples analyzed for grain-size distributions were taken from core intervals immediately
1 Fisher, A., Davis, E.E., and Escutia, C. (Eds.), 2000. Proc. ODP, Sci. Results, 168: College Station TX (Ocean Drilling Program). 2 Department of Geological Sciences, University of Missouri, Columbia MO 65211, USA. Correspondence author:
[email protected] 3 Earth Sciences Department, University of California, Santa Cruz, Santa Cruz CA 95064, USA.
adjacent to those of the physical properties specimens. The main purposes of this report are to show how the grain size and physical properties data are interrelated, and to determine how lithology might affect hydrologic properties of the sedimentary cover.
LABORATORY METHODS The chores of sample preparation were divided equally between labs at the University of Missouri and University of California, Santa Cruz. Because of subtle differences in procedure, several samples were split and prepared in both labs to test for reproducibility (Table 1). The first step in sample preparation was to remove pore water by freeze drying. Dried samples (typically 10–20 g) were stored in a desiccator to prevent moisture from being absorbed, and the dry weights were recorded. The samples then were transferred to 600-mL beakers and immersed in hydrogen peroxide to digest organic matter. After at least 24 hr of digestion and periodic stirring, 250 mL of sodium hexametaphosphate (Calgon) solution (4 g per 1000 mL deionized water) were added to each beaker to assist disaggregation and prevent clay flocculation. After sitting in Calgon solution for at least 12 hr, the beakers were immersed in an ultrasonic bath for 5–10 min to enhance disaggregation further. Suspensions were washed through a 63-µm screen to separate sand-sized grains from silt and clay. Each sand portion was collected, dried in an oven, and weighed. Each fraction 4 µm and
